ScienceShot

ScienceShot: Irrigation Raises Sea Levels

Melting glaciers and icecaps aren't the only things raising sea levels—so is watering your lawn. According to a study published in Geophysical Research Letters, irrigation and other ground water extractions pull immense volumes of water from deep underground and dump it into the oceans via runoff into streams, rivers, and other waterways. Using information gathered worldwide and then extrapolating known trends to regions where data is sparse or missing altogether, one researcher estimates that, over the last century, humans pumped more than 4500 cubic kilometers of water from the ground—enough to boost sea level by 12.6 millimeters, or more than 6% of the overall increase measured during that period. In recent years, when ground water extractions have skyrocketed, the contribution was even larger: From 2000 to 2008, humans pumped on average about 145 cubic kilometers of ground water from aquifers each year—enough to raise sea levels by about 0.4 millimeters annually, or about 13% of the measured amount during that interval. Of the remaining 87% of sea level rise, some studies suggest that about half results from the melting and runoff of land-based ice, with the other half stemming from the thermal expansion of the oceans due to an increase in their temperature, particularly in their surface waters.